A biomass resource is an organic resource which is produced by photosynthesis from water, carbon dioxide, and solar energy and can be utilized as an energy source or a chemical raw material. If the production volume of the product produced from a biomass resource and the amount of the product that is used can be harmonized, a biomass resource is a renewable resource that can be used without increasing carbon dioxide emissions.
Biomass refers to “waste biomass”, which is organic waste that is discharged as unnecessary substances in processes involving life or industrial activities, “unused biomass” such as non-edible parts of crops plowed into farmland or left in the forest (for example, the stems/leaves of corn) or timber from forest thinning, “resource crops”, which are plants grown in currently fallow land or unused land for the purpose of obtaining material and energy resources rather than producing food or wood, “new crops” that are resource crops having functions such as productivity improved by breeding using conventional methods and genetic recombination techniques, and the like.
Biomass is composed of components such as cellulose, hemicellulose, lignin, and intracellular components, and the component ratio differs depending on the type of biomass. For example, wood-based biomass is composed of approximately 50% cellulose, from 20 to 25% hemicellulose, from 20 to 25% lignin, and approximately 5% intracellular components. These components can be used industrially.
For example, cellulose can be used as a paper pulp or a dissolving pulp. Further, since cellulose is a polymer of glucose, it is possible to obtain glucose or cellooligosaccharide from cellulose. Glucose can be used as a raw material for the fermentation of ethanol or lactic acid, and cellooligosaccharide can be used as a functional food. A sugar alcohol obtained by reducing glucose (sorbitol) is widely used as a sweetener providing a cool sensation and has attracted attention in recent years as a biomass-derived plastic raw material (Non-Patent Document 1).
On the other hand, hemicellulose is a polymer heteropolysaccharide composed of xylan, mannan, galactan, or the like and is formed from xylose, arabinose, mannose, galactose, or the like. A monosaccharide such as xylose or arabinose or an oligosaccaride such as a xylooligosaccharide can be obtained from hemicellulose. In addition, like glucose, monosaccharides such as xylose can also be used as a raw material for fermentation. Xylitol, which is obtained by reducing xylose, is incorporated into infusions for diabetic patients and into chewing gum and the like as a sweetener less likely to cause tooth decay. Mannitol, which is obtained by reducing mannose, is also used as a sweetener, and a diuretic effect, an effect of lowering intracranial pressure by opening the brain barrier, and an effect of promoting the transport of drugs into the brain have been reported (Non-Patent Document 2).
Further, a pentose such as xylose or arabinose can be converted to a furfural, and a hexose such as glucose or mannose can be converted to 5-hydroxymethylfurfural. These furfurals can be used as intermediates of pharmaceuticals, raw materials for plastics, or raw materials for furfuryl alcohols (raw materials for furan resins). 2,5-furandicarboxylic acid, which is obtained by oxidizing 5-hydroxymethylfurfural, is expected to be used as a polyester monomer as an alternative substance to terephthalic acid. In addition, 2,5-dimethylfuran, which is obtained by the hydrogenolysis of 5-hydroxymethylfurfural, is expected to be used as an alternative fuel to gasoline. The United States Department of Energy cites 12 types of chemical products such as xylitol, sorbitol, and 2,5-furandicarboxylic acid as chemical products that can be developed from biomass resources using bio-processes as key technologies and that are highly likely to be established as an industry (Non-Patent Document 3).
The components constituting a biomass can be decomposed and extracted by subjecting the biomass to pressurized hot water treatment. Pressurized hot water is in a high-temperature, high-pressure liquid state that has a temperature of 100 to 374° C. and is pressurized to saturated steam pressure or higher. The components of the biomass can be separated by utilizing the difference in reactivity of the biomass components with respect to the pressurized hot water. For example, it has been reported that when the temperature of the pressurized hot water is from 100 to 140° C., it is possible to recover intracellular useful components (tannins, terpenes, and organic acids) or water-soluble lignin. In addition, it has been reported that when the temperature of the pressurized hot water is from 140 to 230° C., it is possible to recover oligosaccharides derived from hemicellulose or monosaccharides such as xylose, arabinose, mannose, and galactose (Patent Document 1, Non-Patent Documents 4 to 6).
Of the types of pressurized hot water treatment described above, a pressurized hot water treatment used as a pre-process of a kraft cooking method when producing dissolving pulp is called a prehydrolysis step. In order to produce dissolving pulp from biomass, it is necessary to selectively remove the lignin and hemicellulose in the biomass to enhance the cellulose purity. Prehydrolysis in the production of pulp is performed under conditions under which the decomposition of cellulose is suppressed and only hemicellulose is decomposed. In the prehydrolysis step, simply adding water to the biomass and heating causes the desorption of acetyl groups in hemicellulose and produces acetic acid, which causes the biomass to become acidic and promotes acidic hydrolysis. Mannose, glucose, and galactose which are hexoses, and xylose and arabinose, which are pentoses, are contained as constituent sugars in the hemicellulose.
In the prehydrolysis step, oligosaccharides consisting of the sugars described above are produced when hemicellulose is hydrolyzed. In addition, monosaccharides are produced when the hydrolysis of the oligosaccharides progresses further. Among these sugars, xylose and aribinose, which are pentoses, are converted to furfurals by a dehydration reaction of three molecules of water (Non-Patent Document 7). In the hydrolysate (solid content) after the biomass is subjected to prehydrolysis, the lignin and hemicellulose remaining in the hydrolysate in the kraft cooking process of a subsequent stage are removed, and high-purity cellulose (dissolving pulp) is obtained by further performing bleaching treatment in the next step.
As described above, the primary objective of the prehydrolysis step is to efficiently produce dissolving pulp (cellulose), so prehydrolysis is performed under conditions suitable for the production of dissolving pulp. Water is typically added to the raw material chips (absolute dry weight) at a liquid ratio of approximately 2 to 5 and processed for one to several hours at 150° C. to 180° C. In addition, suitable prehydrolysis conditions are set in accordance with the type of the raw material and the quality of the target dissolving pulp. Accordingly, the ratio of oligosaccharides, monosaccharides, and furfurals contained in the reaction liquid after prehydrolysis is not the target ratio, so there is a problem in that it is not possible to efficiently produce the components of interest. If it were possible to arbitrarily control the production ratio of oligosaccharides, monosaccharides, and furfurals derived from hemicellulose, it would be possible to produce the components in accordance with the demand thereof. When aiming for practical application at an industrial scale, this is advantageous from an economical perspective since it is possible to efficiently produce only the necessary components of interest. Further, if it were possible to improve the production efficiency of the oligosaccharides, monosaccharides, and furfurals derived from hemicellulose in the prehydrolysis conditions used for the dissolving pulp, this would enable the production of dissolving pulp as well as the practical application of the monosaccharides, oligosaccharides and furfurals contained in the hydrolysis solution at an industrial scale.
As techniques for controlling the production volume of products by means of pressurized hot water treatment using biomass as a raw material, a method of controlling the ratio of the production volumes of the hemicellulose degradation product and the cellulose degradation product by changing the amount of pressurized hot water supplied to the biomass (Patent Document 2), a method of primarily decomposing hemicellulose in a first hydrolysis step and primarily decomposing cellulose in the residue of the first step in a second hydrolysis step (Patent Document 3), and a method of decomposing and extracting hemicellulose by subjecting the biomass to pressurized hot water treatment at 140 to 230° C. and then decomposing and extracting cellulose by subjecting the biomass to pressurized hot water treatment at the temperature not less than the temperature described above (Patent Document 4) have been reported. However, there has yet to be disclosed a technique for controlling the production ratio of the respective components of the monosaccharides, oligosaccharides, and furfurals simultaneously obtained when the biomass is hydrolyzed.
In addition, as a method for manufacturing xylose and xylooligosaccharides from a biomass raw material, a method of treating a water-insoluble residue, which is prepared by removing components extracted from a xylane-containing natural product with hot water at a temperature of at least 110° C. and at most 140° C., with hot water at a temperature of at least the above treatment temperature and at most 200° C. (Patent Document 5) has been reported. However, there has yet to be disclosed a report related to a technique for increasing the productivity of the respective components of the monosaccharides, oligosaccharides, and furfurals simultaneously obtained when the biomass is prehydrolyzed. Moreover, there has also yet to be disclosed a report related to a technique for efficiently separating and recovering the monosaccharides, oligosaccharides, and furfurals simultaneously obtained when the biomass is prehydrolyzed.
Hydrolysis methods are typically classified as a batch method or a continuous method. In a batch method, after a mixture of biomass and an aqueous solution is supplied to a hydrolysis device, the lid of the hydrolysis device is sealed and heated to perform hydrolysis. After hydrolysis, the operation of the device is temporarily suspended, and a solution containing the reaction product is separated and recovered. As a method of producing furfurals with a batch method, a method of adding biomass to a digester, sealing the lid of the digester, performing a hydrolysis reaction by heating the digester for 1 to 2 hours at 160 to 170° C., and recovering the furfurals contained in the gaseous phase has been reported. In this method, it has been reported that the furfural concentration in the aqueous solution recovered from the gaseous phase is approximately 3 to 6 wt. % and that purification can be easily performed by distillation (Non-Patent Document 8). However, in the batch method, after the first run, it is necessary to stop the hydrolysis device and then restart the operation, so it is not possible to process large quantities of biomass, which is problematic in that the production efficiency is poor in comparison to a continuous method.
On the other hand, in a continuous method, a mixture of biomass and an aqueous solution is supplied to a heated hydrolysis device, and the reaction products are continuously recovered. In the continuous method, it is possible to process large quantities of biomass in a short period of time, so there is the merit that the production efficiency of reaction products is high in comparison to the batch method. However, the continuous method has a problem in that the furfural concentration in the gaseous phase becomes low since furfurals are dissolved in the aqueous phase. In order to increase the furfural recovery efficiency, it is necessary to increase the furfural concentration in the gaseous phase as much as possible since furfurals can be recovered from the gaseous phase.
In order to achieve the practical application of furfural production from biomass at an industrial scale, a problem of the continuous hydrolysis method is to reduce the production cost by establishing an efficient furfural recovery method. As a system for producing furfurals from lignocellulose raw materials, a method of performing digestion on wood chips with a continuous digestion device using a lower aliphatic alcohol as a solvent and then recovering by-products such as a furfural from a black liquor produced as a by-product of pulp production has been reported (Patent Document 6). In this system, after the black liquor following digestion is transferred to a flash tank and separated into a gaseous phase (fraction containing ethanol) and a liquid phase (fraction containing furfural), the ethanol used as a drug solution for digestion is recovered from the gaseous phase. On the other hand, the furfural concentration in the liquid phase is from 0.2 to 0.8%, and furfural is concentrated in a subsequent step. In order to efficiently produce furfural, it is preferable to increase the furfural concentration (yield) in the preceding step as much as possible. Presently, there is no report of an economically applicable and efficient furfural separation and recovering method related to furfural production using biomass as a raw material. Accordingly, there is a need for the establishment of a continuous method that increases the furfural concentration in the gaseous phase as much as possible. In addition, there is also a need for the development of a method for efficiently recovering the monosaccharides and oligosaccharides contained in the hydrolysis solution at the same time as furfural production.